1. Field of the Invention
The present invention relates to an image fixing device which performs heat fusing of an unfixed toner image in an image formation apparatus utilizing the electro-photographic method, such as a copying machine, a printer or a facsimile apparatus and in particular relates to an image fixing device adopting a belt nip method.
2. Discussion of the Related Art
FIG. 8 shows an image fixing device proposed by inventors of the present invention and disclosed by Japanese Patent Application Unexamined Publication No. Hei. 5-150679 (1993). In the figure, the reference number 1 indicates a heat fusing roller rotatably disposed. The heat fusing roller 1 is made by forming a coating layer 4 of an elastic material on the surface of a core 3 which is a cylinder made of metal with high thermal conductivity, for example, aluminum. The coating layer 4 comprises an under coating layer 20 of HTV (High Temperature Vulcanization) silicone rubber formed directly on the surface of the core 3 and a top coating layer 21 of RTV (Room Temperature Vulcanization) silicone rubber formed to cover the under coating layer 20.
As a heat source, a halogen lamp 5 is disposed inside of the core 3. A temperature sensor 6 is disposed at the surface of the heat fusing roller 1 to measure the surface temperature of the heat fusing roller 1. In accordance with a measurement signal of the temperature sensor 6, a temperature controller (not shown in the figure) is operated to control the halogen lamp 5, whereby temperature of the surface of the heat fusing roller 1 is controlled to be a predetermined value. As releasing agent, silicone oil is supplied to the surface of the heat fusing roller 1 by an oil supplying device 9; therefore offset of a part of toner 8 to the heat fusing roller 1 is prevented in fixing an unfixed image of toner 8 to a sheet 7.
A pressure roller 25 is pressed to the heat fusing roller 1 by a compressed coil spring 26, and an assist pressure roller 29 is pressed to the heat fusing roller 1. Rollers 22, 23 and 24 are mounted to support an endless belt 15 so that part of the endless belt 15 is disposed between the heat fusing roller 1 and the pressure rollers 25 and 29.
The pressure roller 25 is contacted to the heat fusing roller 1 with pressure at a position higher than that of the roller 24, whereby the endless belt contacts to the heat fusing roller 1 and an area where the heat fusing roller 1 and the endless belt 15 contact each other is made to be a nip portion (hereinafter, referred to as a belt nip) for passing the sheet 7 therethrough. As according to rotation of the heat fusing roller 1 and the endless belt 15 in a direction indicated by an arrow, the sheet 7 is forwarded between rotating heat fusing roller 1 and endless belt 15. When the sheet 7 is passed through the belt nip, unfixed toner 8 is fixed to the sheet 7 by pressure applied to the belt nip and heat provided by the halogen lamp 5 through the heat fusing roller 1.
With such belt nip method construction, because the sheet 7 is heated for the time corresponding to the width of the belt nip (length of the part of the belt pressed by the pressure roller 25 and the assist pressure roller 29 in the figure), it is possible to securely obtain sufficient fixing time even if the carrying speed of the sheet 7 is increased in comparison with the case where only the heat fusing roller 1 and the pressure roller is press-contacted each other without using the endless belt 15. In the case of the same carrying speed, the heating time in the belt nip method is longer than that in a method in which the endless belt is not used, and therefore large amount of heat can be supplied to the toner. Consequently, the belt nip method is particularly suitable for image fixing by a color copying machine which produces desired colors by multi-layered toner.
According to the image fixing device of the present invention, the sheet 7 can be stripped from the heat fusing roller 1 without stripping means such as stripper fingers used in the ordinary image fixing device comprising only the heat fusing roller and the pressure roller (hereinafter, this is referred to as self-stripping). Consequently, a thin recording sheet which is soft and is difficult to be stripped or a recording sheet to which a large amount of toner is fused can be stripped by self-stripping.
During the study of self-stripping method for a thin recording sheet, that is, a method of reducing adhesive force between the fixing roller and melted toner, the inventors of the present invention discovered the following phenomenon.
The adhesive force at an interface of melted toner and the surface of the fixing roller is not only determined by their surface-chemistrical material properties, but also influenced largely by strain of the fixing roller. That is, the adhesive force between toner and the surface of the fixing roller is reduced when a state that the surface of the fixing roller is already strained and melted toner is fused thereon is changed to another state that strain of the surface of the fixing roller is restored in a moment.
To be concrete, if the surface of the fixing roller is coated with material which can be elastically deformed and strained by the load applied from the outside, for example, heat-resisting elastic material such as silicone rubber or fluororubber, the adhesive force between toner and the surface of the fixing roller is severely reduced at the moment when the strain of the surface of the fixing roller made by being contacted and applied pressure by a hard roller of small diameter for fixing is restored at the exit point of the nip; accordingly, self-stripping of the recording sheet is performed with ease.
The mechanism of reduction of the adhesive force is not yet made clear, but inventors of the present invention consider this as follows.
When the pressure is applied, the surface of a soft roller which is coated with elastic material is deformed; therefore it is in contact with toner in the state of being strained. When the pressure is removed suddenly from the soft roller which is strained, strain is restored and the soft roller is returned to the former state of shape. At this time, a slight slip occurs at the interface of the toner and the roller, and the slight slip is assumed to be the cause of the adhesive force reduction effect.
In the case of the hard roller whose surface is not deformed, no slight slip occurs. Therefore, adhesive force reduction effect cannot be obtained. The present invention applies this idea to the belt nip method.
Other image fixing devices adopting belt nip method are disclosed by Japanese Patent Application Unexamined Publications Nos. Sho. 52-69337 (1977), Sho. 60-151677 (1985), Sho. 60-151681 (1985), Sho. 62-14675 (1987), Japanese Utility Model Application Unexamined Publications Nos. Sho. 60-104852 (1985) and Hei. 2-30961 (1990).
However, the image fixing devices adopting the belt nip method described above have the following problems:
1) Because the pressure roller 25 and the assist pressure roller 29 pressing the endless belt 15 toward the heat fusing roller 1 rotate as according to the movement of the endless belt 15, the pressure roller 25 and the assist pressure roller 29 carry away a large amount of heat from the heat fusing roller 1. It is uneconomical, and besides, the surface temperature suddenly drops, whereby fixing capability of toner is influenced badly.
2) Because the coating layer 4 is slightly strained by the pressure-contacting force of the pressure roller 25, if supposing that peripheral velocity of the heat fusing roller 1 is V.sub.0, and strain of the coating layer 4 in the circumferential direction is .epsilon., the velocity of the strained portion V.sub..epsilon. is represented as follows (See FIG. 9): EQU V.sub.68 =V.sub.0 (1+.epsilon.)
In the belt nip, the frictional coefficient between the sheet 7 and the heat fusing roller 1 is considered to be constant anywhere in the sheet 7. The force of the endless belt 15 pressing the sheet 7 to the heat fusing roller 1 by itself is not so large, and accordingly, it is considered that the largest frictional force is generated at the portion in contact with the pressure roller 25 which applies the largest load. Moreover, elongation of the sheet 7 is considered to be extremely small. Therefore, the sheet 7 is carried at the velocity V.sub.P1 which is close to the velocity V.sub..epsilon. The part of the heat fusing roller corresponding to the belt nip moves at the peripheral velocity V.sub.0 and the sheet 7 is carried at the velocity V.sub.P1, and consequently, the following velocity difference occurs between them: EQU .epsilon.V.sub.0 .apprxeq.V.sub.P1 -V.sub.0
Owing to this velocity difference in the belt nip, the unfixed image of toner 8 is disordered, which results in image displacement in an obtained fixed image.
For this reason, in the device disclosed by Japanese Patent Application Unexamined Publication No. Hei. 5-150679, the assist pressure roller 29 is disposed at the upper side of the pressure roller 25 in the direction of movement of the sheet 7 and pressed to the heat fusing roller 1 by a spring which is not shown in the figure. Therefore, even if the end of the sheet 7 reaches the pressure roller 25 and is going to be carried at the velocity V.sub.P1 which is close to the velocity V.sub..epsilon., it is possible to carry the sheet 7 at the peripheral velocity V.sub.0 by pressing the following portion of the sheet 7 to the heat fusing roller 1 by the assist pressure roller 29 to prevent occurrence of difference in velocities between the heat fusing roller 1 and the sheet 7 which causes image displacement. However, it is difficult to securely avoid the image displacement only by means of mounting the assist pressure roller 29. If the assist pressure roller 29 is mounted, the device will be complex and bulky, and besides, the number of the components increases, thus being uneconomical.
3) If the sheet 7 and toner 8 is heated in the belt nip, air and water vapor swell and vaporize from the heated sheet 7 or toner 8. Such air and water vapor turn to bubbles in the belt nip, to be more detail, between the sheet 7 and the heat fusing roller 1 or the endless belt 15, until the sheet 7 passes through the belt nip.
Apart from portions pressed by the pressure roller 25 and the assist pressure roller 29, pressure sufficient to fix the toner 8 to the sheet 7 cannot be applied between the heat fusing roller 1 and the endless belt 15 because of interposition of the bubbles. Besides, in the case where the sheet 7 is in the belt nip and the toner 8 is not fixed completely, sometimes unfixed toner 8 is undesirably moved by the bubbles vaporized from the sheet 7. In a method using a heat fusing roller and a pressure fixing roller without an endless belt, such inconvenience hardly occurs because of a small nip width, but in the belt nip method there is a large possibility of displacement of toner image by the bubbles since the nip width is large. In the practical use, there occurs a problem of image bleeding or displacement considered to be caused by the bubbles.